Glass articles of complex shapes, which exhibit close dimensional accuracy and excellent surface quality, are virtually impossible to produce utilizing conventional hot glass forming methods. Shapes of some complexity and dimensional accuracy can be formed through hot glass molding, but the use of a mold severely limits surface quality. Good surface quality can be secured only by limiting the glass surface to air contact, but that technique permits the production of simple shapes only. Furthermore, shapes with sharp corners cannot be made via hot forming processes. Hence, grinding and polishing are required to obtain sharp corners, high dimensional accuracy, and excellent surface quality. But, unless the desired shape is relatively simple, grinding and polishing involve laborious and expensive hand operations.
Injection molding is a common plastics molding process which is capable of producing finished parts of complex, intricate shapes with high dimensional accuracy and excellent surface quality. FIG. 1 schematically depicts the general injection molding process as applied to plastics forming. As is represented therein, fluid plastic is injected into a closed, stationary mold. The fluid solidifies, the mold is opened, the plastic body ejected, and the cycle is then repeated. The injection molding apparatus represented in FIG. 1 is equipped with a screw pump. A screw pump is efficient because it permits melting, mixing, and de-airing to be combined with semi-continuous pumping. The screw pump is used solely to fill the cylinder, with the actual injection being accomplished by driving the screw forward in like manner to a piston. Of course, simple piston and cylinder injection molders are known, but their use is no longer common.
The size of an injection molding machine is generally determined by two factors: (1) the volume of plastic that can be injected; and (2) the clamping force holding the mold closed. Injection pressures most typically range between about 10,000 to 20,000 psi, the pressure used striking a balance between filling complex molds quickly and avoiding flash (plastic forced into the parting line of the mold) which can occur when the mold is forced open by the plastic. Thermoplastic materials are customarily injected at whatever temperature yields a zero shear viscosity of approximately 10.sup.4 poises. High temperatures are desirable to promote fast injection and to prevent freezing of the plastic before the mold is full. The mold is commonly at a lower temperature than the incoming plastic. Temperatures which are too high, however, lead to flash or thermal degradation of the plastic.
It has been repeatedly demonstrated that injection molding machines have the capability of forming articles of plastics over a wide range of operating conditions. The commercial emphasis has been directed to tailoring the operating conditions with the view of minimizing the cycle time and optimizing the process. The cycle time of a well-adjusted apparatus is normally primarily dependent upon the cool down time of the plastic article. Cycle times of about ten seconds are quite common.
The hydration of alkali metal-containing silicate glass bodies to impart thermoplastic properties thereto is well-known to the prior art. For example, U.S. Pat. No. 3,498,802 discloses the hydration of alkali metal silicate glass powders to produce thermoplastic materials and hydraulic cements. In mole percent on the oxide basis, the anhydrous glass powders consisted essentially of 80-94% SiO.sub.2 and 6-20% Na.sub.2 O and/or K.sub.2 O, the sum of those components constituting at least 90 mole percent of the total composition. Numerous compatible metal oxides such as Al.sub.2 O.sub.3, BaO, B.sub.2 O.sub.3, MgO, PbO, and ZnO can, optionally, be included but CaO and Li.sub.2 O are preferably avoided. The hydration procedure contemplates contacting the anhydrous glass powders with a gaseous environment containing at least 50% by weight water at a pressure of at least one atmosphere and a temperature customarily within the range of about 100.degree.-200.degree. C. This treatment in the steam atmosphere is continued for a sufficient length of time to develop at least a surface layer on the powders containing up to 30% by weight H.sub.2 O. Temperatures of 80.degree.-120.degree. C. are observed as causing the hydrated powders to become adhesive and cohesive thereby enabling the thermoplastic material to be shaped utilizing such conventional forming methods as pressing, rolling, extrusion, and injection molding.
U.S. Pat. No. 3,498,803 describes the hydration of certain glass and glass-ceramic compositions to yield articles exhibiting plastic or rubbery characteristics. The anhydrous glass compositions operable in the invention consist essentially, in mole percent on the oxide basis, of about 6-40% Na.sub.2 O and/or K.sub.2 O and 6-94% SiO.sub.2, the sum of those ingredients constituting at least 85 mole percent of the total composition. BaO, B.sub.2 O.sub.3, MgO, PbO, P.sub.2 O.sub.5, and ZnO are mentioned as possible additions with CaO and Li.sub.2 O being preferably absent. The hydration process comprehends contacting the anhydrous glass with a gaseous environment containing at least 50% by weight H.sub.2 O at a pressure of at least one atmosphere and a temperature conventionally within the range of about 80.degree.-200.degree. C. This treatment in the steam atmosphere is continued for a period of time sufficient to develop at least a surface layer on the glass articles containing between about 5-35% H.sub.2 O.
U.S. Pat. No. 3,912,481 is drawn to a particular two-step method for steam hydrating alkali metal containing silicate glasses to effect thermoplastic properties therein. The process involves first hydrating an anhydrous glass body consisting essentially, in mole percent on the oxide basis, of about 3-25% Na.sub.2 O and/or K.sub.2 O and 50-95% SiO.sub.2, the sum of those components constituting at least 55% of the total composition, in a steam atmosphere having a relative humidity of at least 75% at a temperature of at least 100.degree. C. and, possibly, up to 600.degree. C. for a sufficient length of time to develop at least a surface portion on the body which is saturated with water. Thereafter, the saturated article is dehydrated via contact with a steam atmosphere wherein the relative humidity is less than 90% of that employed in the hydration step for a length of time sufficient to reduce the water content within the body but leaving an amount therein effective to impart the desired themoplastic properties thereto. The method permits close control to be exercised over the amount of water retained within the glass body. Such compatible metal oxides as Al.sub.2 O.sub.3, BaO, B.sub.2 O.sub.3, CaO, MgO, PbO, and ZnO are noted as being advantageously included.
The patent also discloses the shaping of the so-produced thermoplastic materials utilizing standard compression molding techniques. For example, particles of hydrated glass were run into a mold and then formed into a bulk shape under pressure and temperatures up to 500.degree. C.
U.S. Pat. No. 3,948,629 describes the hydration of alkali metal-containing silicate glass particles in an aqueous solution having a pH less than 6 to develop at least a surface layer on the particles having a water content ranging up to 36% by weight. The operable glass compositions are stated to be the same as those disclosed in U.S. Pat. No. 3,912,481 above. The method involves contacting the glass particles with the solution at a temperature greater than 100.degree. C. and, possibly, up to 374.degree. C. and at a pressure in excess of 20 psi and, possibly, up to 3200 psi. for a sufficient length of time to cause H.sub.2 O to be absorbed within the glass in an amount effective to impart thermoplastic properties thereto.
This patent also discloses the shaping of the hydrated particles into bulk shapes via compression molding techniques relying upon the thermoplastic behavior displayed by the particles as a result of the absorbed water. Temperatures up to 500.degree. C. are stated to be suitable.
U.S. application Ser. No. 583,606, filed June 4, 1975 by H. E. Meissner, J. E. Pierson, R. D. Shoup, and S. D. Stookey, and now abandoned, describes a method for producing microporous silicate glass bodies from precursor sodium and/or potassium hydrosilicate glass bodies, i.e., glass bodies containing about 5-50% by weight water within their structures. Where desired, the microporous bodies can be fired to consolidation as essentially anhydrous, solid glass articles.
The crux of that invention resides in replacing the Na.sup.+ and/or K.sup.+ ions in the precursor hydrosilicte glass, either partially or totally, with protons and/or other monovalent and polyvalent cations. Where desired, the other chemical species can subsequently be incorporated into the body of the glass in those instances where the species are capable of being sorbed on the porous glass by physical and/or chemical forces. Such chemical species can include polyvalent cations, anions, and neutral molecular species, either organic or inorganic, and complex ions such as metal ammine complexes. The replacement reactions with the Na.sup.+ and/or K.sup.+ ions and the sorption of the other chemical species take place in an aqueous solution medium. The same solution may contain the additional chemical species to be sorbed or the glass can be contacted with a second solution containing the additional species.
Because the precursor body is hydrosilicate glass and the exchange reaction is carried out in an aqueous solution, the resultant exchange of ions, normally leading to a dealkalization of the glass, can readily take place throughout the entire glass body. Thus, the exchange of ions is not limited to a surface effect but results in a microporous glass body. It is this microporosity which permits the sorption of various chemical species within the interior of the glass, thereby promoting the production of silicate glasses of widely-differing overall compositions, but which compositions can be substantially uniform throughout the body.
The precursor hydrosilicate glass body can be prepared in two ways: (1) an anhydrous sodium and/or potassium silicate glass can be hydrated, e.g., in the manners described in U.S. Pat. Nos. 3,498,802, 3,498,803, 3,912,481, or 3,948,629, supra; or (2) through the drying and shaping of aqueous sodium and/or potassium silicate solutions. Since this latter method does not involve an initial glass melting step, it has the economic advantage of a less energy-intense process. Either process permits the shaping of the hydrosilicate material via compression molding techniques. Normally, the very nature of the material utilized in the second process leads to greater ease in shaping.
In general, the initial sodium and/or potassium hydrosilicate glass body will have an anhydrous composition consisting essentially, by weight on the oxide basis, of about 10-60% Na.sub.2 O and/or K.sub.2 O and 40-90% SiO.sub.2, the sum of Na.sub.2 O and/or K.sub.2 O+SiO.sub.2 constituting at least 75% of the total composition. Up to 25% by weight total of such compatible metal oxides as BaO, Al.sub.2 O.sub.3, GeO.sub.2, SnO.sub.2, As.sub.2 O.sub.3, B.sub.2 O.sub.3, SrO, MgO, ZnO, ZrO.sub.2, CaO, Sb.sub.2 O.sub.3, and PbO, and such anions as halides, carbonates, chlorates, bromates, iodates, cyanides, sulfides, borates, phosphates, aluminates, plumbates, zincates, chromates, germanates, stannates, antimonates, and bismuthates may be present.
The use of compression molding to form bulk shapes from hydrated granules, from dried silicate solutions, or to reform pre-formed hydrated bodies has, however, several inherent limitations:
(1) pre-formed bodies are limited in thickness dimension because of the long time required for hydration and/or dehydration;
(2) granules require relatively short hydration times but yield molded articles with visible grain boundaries;
(3) molded and reformed articles are restricted to shapes with a region of reasonably constant cross-section;
(4) accurate measurement of charging materials is required to insure control of the size and shape of the article produced;
(5) the molds customarily have precision sliding surfaces in contact with the hydrosilicate material and, therefore, are subject to wear; and
(6) the molds require dynamic sliding seals.